Fluoropolymers such as polytetrafluoroethylene, polyvinylidene fluoride are encountered in numerous technical fields.
Polytetrafluoroethylene (PTFE), a synthetic fluoropolymer of tetrafluoro-ethylene is used as a non-stick coating for pans and other cookware; as an insulator in cables and connector assemblies and as a material for printed circuit boards used at microwave frequencies. It is often used in containers and pipework for reactive and corrosive chemicals.
Polyvinylidene fluoride (PVDF) is available as piping products, sheet, tubing, films, plate, membranes, powder coatings, foams and wiring insulator. It can be injected, molded or welded and is commonly used in the chemical, semiconductor, photovoltaic panels, as well as in lithium ion batteries.
A step in which the fluoropolymer is solubilized is sometimes present in the different applications, particularly when the fluoropolymer is used in batteries, or to make membranes for water filtration.
With the increasing demand for battery-powered electronic equipment, there exists a corresponding increase in demand for rechargeable electrochemical cells having high specific energies.
Lithium-ion secondary batteries have high voltage and high capacity compared to conventional nickel cadmium secondary batteries. In particular, when lithium transition metal composite oxides such as LiCoO2 and LiMn2O4 are used as cathode active material and carbonaceous materials such as graphite and carbon fiber are used as anode active materials, high voltage and high capacity can be achieved, and side effects such as short circuits do not occur. Thus, lithium secondary batteries are widely used as power sources for mobile electronic devices such as cellular phones, notebook computers, digital cameras, etc.
Lithium secondary batteries are generally prepared by applying on a metal film a slurry consisting of an active material and a polymeric binder, drying the slurry and pressing the film. Although various resins have been used as binder, fluorine-based resins such as polyvinylidene fluoride (PVDF), which adheres well to the material current collector and active material, is commonly used.
To prepare the slurry, the polymeric binder is usually dissolved in a solvent to form a solution having from about 1 to 15% binder in solvent. Binder solutions are typically formulated with N-methyl pyrrolidone (NMP). NMP is considered to be most effective.
However, the safety risks for the operators and the environment are a permanent concern. NMP will be labeled as Mutagen Cat 2/Reprotoxic R61 from June 2009 in Europe and NMP is subject to report in the Toxic Release Inventory (SARA title III section 313). There is a need for other solvents, presenting a good safety and/or environment profile.
In CN1277236-A and CN1120210-C, an adhesive formula is developed which consists of polyvinylidene fluoride resin 3-15 wt %, N-methyl pyrrolidone or dimethyl acetamide 85-95 wt %, and γ-aminopropyl triethoxy silicone, γ-propyl methacrylate trimethoxy silicone or ethylamino amidopropyl trimethoxy silicone as coupling agent 1-5 wt %.
US2005048368-A1, JP2005072009-A, CN1591939-A, KR2005023179-A and CN100411232-C relate to a separator formula; the employed organic solvent is consisting of dimethylformamide, dimethylsulfoxide, dimethylacetate, acetone, and/or N-methyl-2-pyrrolidine. Dimethylacetamide, dimethylformamide are also classified as CMR (carcinogen/mutagen/reprotoxic).
KR2003047038-A develop composite binder for lithium battery. The composite electrode binder comprises polyvinylidene fluoride (PVDF) and polyimide. Preferably the polyimide is 20% polyimide solution prepared by mixing pyromellitic dianhydride and 4,4′-diaminophenyl ether in the ratio of 1/1 and dissolving the mixture in N-methylpyrrolidone.
JP2002246029-A, WO200273720-A2, AU2002257642-A1 and AU2002257642-A8 describe a new binder composition comprising fluoro-resin-A which dissolves in a specific organic solvent and resin-B which totally or partially does not dissolve in the organic solvent. Fluoro-resin-A comprises fluoro-polymer-A1 whose 8 wt. % N-methylpyrroldinone solution has a viscosity of 0.3-20 Pa·s and fluoro-polymer-A2 in which polar groups are incorporated. The weight ratio of NB is 99/1 to 1/99.
Poly(vinylidene fluoride-hexafluoropropylene)-based membranes for lithium batteries are disclosed in Journal of Membrane Science (2008), 310(1+2), 349-355. Poly(vinylidene fluoride-hexafluoropropylene) (PVdF-HFP) copolymer membranes are prepared by phase inversion with poly(ethylene glycol) as additive and with THF, acetone or DMF as solvents.
Mechanical and electrical properties of poly(vinylidene fluoride-tetrafluoroethylene-propylene)/Super-S carbon black swelled in liquid solvent as an electrode binder for lithium-ion batteries are disclosed in Journal of Applied Polymer Science (2004), 91(5), 2958-2965.
In the different applications, there is a need for finding solvents with a better health, safety and environment profile in order to prepare solutions of fluoropolymers.